A key aspect in all modern radio transceivers is the ability to synthesize the frequency of the communication channel of interest. Adjacent communication channels can be spaced by only tens of kilohertz, while operating at several gigahertz. Therefore, the frequency synthesis must be extremely precise, otherwise different transceivers will interfere with each other. In the fifth generation of mobile communication, 5G, frequencies above 30 GHz, so-called millimeter-wave (mm-wave) frequencies are expected to play a prominent role. For the synthesis of these frequencies, the digital phase locked loop (DPLL) has recently gained much attention. A core component of the DPLL is the digitally controlled oscillator (DCO), an oscillator that tunes the frequency discretely. For good performance, the frequency steps of the DCO must be very small, while the total tuning range must be large. The DCO takes a digital word as its input and tunes its frequency discretely.